1. Field of the Invention
The present invention relates to an amplifier circuit.
2. Description of Related Art
CMOS (Complementary Metal Oxide Semiconductor) technology is often used in portable electronic devices in which low power supply voltage and low power consumption are required, and therefore CMOS amplifiers have been proposed from various viewpoints.
FIG. 7 is a simplified schematic view of an amplifier disclosed in Japanese Unexamined Patent Application Publication Nos. 62-160815 and 61-289717 (Patent documents 1 and 2). The amplifier 10 is composed of a CMOS push-pull inverter (hereinafter, simply called “inverter”) 11 and a negative feedback resistor 14. The inverter 11 includes an NMOS transistor 12 and a PMOS transistor 13, which are connected to each other at their drains. The source of the NMOS transistor 12 is connected to a power supply voltage VCC and the source of the PMOS transistor 13 is connected to a ground voltage GND. Furthermore, the gates of both the NMOS transistor 12 and the PMOS transistor 13 are connected to an input voltage VIN, so that they are driven by the input voltage VIN. Furthermore, the negative feedback resistor 14 is connected between the input voltage VIN and the output voltage VOUT of the inverter 11.
The amplifier 10 can produces the maximum voltage gain when both the NMOS transistor 12 and the PMOS transistor 13 are operated in the saturation region.
In the amplifier 10 shown in FIG. 7, the presence of the negative feedback resistor 14 can stabilize the DC bias point in the amplifier 10. Furthermore, the negative feedback resistor 14 can provides another useful effect of reducing the output resistance of the amplifier 10 in a state where the inverter 11 attains a sufficient gain in the operating band.
Furthermore, Japanese Unexamined Patent Application Publication Nos. 8-77779 and 2000-306382 (Patent documents 3 and 4) propose differential amplifiers. FIG. 8 is a simplified schematic view of a differential amplifier disclosed in Patent documents 3 and 4. A differential amplifier 30 shown in FIG. 8 includes two amplifiers 10 each of which is shown in FIG. 7. Because of the differential structure, the differential amplifier 30 can provide a higher gain than the gain produced by the amplifier 10 shown in FIG. 7.